Gallery slow

Slideshow full screen is very slow when changing images. The progress bar appears for a while before the image changes. About 4 or 5 images in I often get a black screen and then the images change at the correct speed and the gallery runs fine.

I have tried on the latest version and have recently rolled back to 2.0.57. All other plugins and WordPress are up to date.

http://www.hartsathome.co.uk/gallery

https://wordpress.org/plugins/nextgen-gallery/